Active optical devices, which are currently being studied for use in future telecommunications and computational systems, require materials that are characterized by large optical nonlinearity. In such materials a light beam will cause a significant change in the optical properties of the medium, altering the propagation of other light beams (cross-modulation) or of itself (self-modulation). Many optical devices further require that the material have a fast response time and low optical loss. Glasses containing high concentrations of heavy metal oxides (HMO) have been found to display a high degree of optical nonlinearity which, due to its electronic nature, is further characterized by a rapid response time. For example, measurements on representative HMO glasses have shown a non-linear refractive index some 50.times. that of vitreous silica. These features, coupled with excellent transmission in both the visible and infrared portions of the electromagnetic spectrum, render these materials ideal for a wide range of optical applications, including those where light induced phase changes, beam steering, optical phase conjugation, and bistability of resonator structures are desired.
Optical devices utilized in these applications require that the optical properties of the active material change in response to an applied optical field, the magnitude of this effect being characterized by a single term called the third order susceptibility tensor, X.sup.(3). The latter can be thought of as a coefficient in a power series expansion of the relationship between the applied electric field, E, and the polarization, P, written schematically as EQU P=X.sup.(1) E+X.sup.(2) EE+X.sup.(3) EEE+. . .
where X.sup.(1) is the linear susceptibility tensor, X.sup.(2) is the second order susceptibility tensor, and so on. X.sup.(1), the linear susceptibility, is related to the linear refractive index, n.sub.o, by X.sup.(1) =(n.sub.o.sup.2 -1)/4.pi..
When an optical field is applied to a material, such as a glass, the optical properties of the material are affected. The magnitude of this effect can be measured, and is characterized in the art by X.sup.(3). Illustrative of this phenomenon is the example where a high-intensity light beam is passed through a nonlinear medium, consequently inducing a change in the index of refraction. For a light beam of intensity I, the index of refraction can be described by the equation EQU n=n.sub.o +.gamma.I
where n is the index of refraction, n.sub.o is the linear (low intensity) index, and .gamma. is the nonlinear refractive index of refraction. The nonlinear index, in turn, is related to X.sup.(3) by the equation EQU .gamma.=[(480.pi..sup.2)/(n.sub.o c.sup.2)]X.sup.(3).sub.1111
where X.sup.(3).sub.1111 is the first diagonal term of the third order susceptibility tensor and c is the velocity of light. In a nonlinear device this change in refractive index is achieved when one light beam is used to manipulate the behavior of another.
One technique that may be used in measuring X.sup.(3) of various oxide glasses, called degenerate four wave mixing (DFWM), has shown that the maximum values of X.sup.(3) are attained in those glasses having the highest concentration of the heavy metals thallium, lead, and bismuth. The ions of these metals, in their normal oxidation state in HMO glasses, i.e., Tl.sup.+, Pb.sup.++, and Bi.sup.+3, have an electronic configuration which is essentially that of an inert gas plus two 6s electrons. Strictly speaking, the electronic configuration of these ions is [Xe](4f.sup.14)(5d.sup.10)(6s.sup.2), but, as the filled 4f and 5d shells as well as the xenon core comprise a spherically symmetric density, the configuration can be abbreviated as 6s.sup.2. These two 6s electrons comprise a stereochemically active lone electron pair in that they occupy a directed orbital, but this orbital is essentially non-bonding in character. The presence of this lone electron pair is responsible for the highly distorted geometry assumed by Tl--O, Pb-- O, and, to a lesser extent, Bi--O coordination polyhedra, and is postulated to be the source of the high values of X.sup.(3) characteristic of HMO glasses.
DFWM measurements on a number of HMO glasses suggest that, of the three heavy metal ions, Tl.sup.+ has the largest overall contribution on an atom-for-atom basis to the net X.sup.(3) of an HMO glass. Thus, oxide glasses with high concentrations of thallium are here identified as being especially well suited for the fabrication of nonlinear optical devices.
The enhanced nonlinearity of the HMO glasses described herein and waveguide structures that may be synthesized from these glasses make them useful in a number of device configurations, some of which can only be implemented in waveguide form (either fiber or planar waveguides), and some of which can also be implemented in bulk-optics form. Such devices include, but are not limited to: nonlinear mode coupling devices, nonlinear interference devices, and optical amplifiers, and areas where optical phase conjugation is desirable.
Nonlinear mode coupling devices operate through changing the coupling of two (or more) modes of a waveguide structure as a result of the third order susceptibility. They include various multi-core couplers and single-core devices where two or more modes of the waveguide structure (such as modes with different polarizations or spatial distributions) have their coupling altered through nonlinear interaction.
In nonlinear interference devices the relative phases of two or more light beams (or even various reflections of a single light beam) are changed by utilizing variations in the optical path length resulting from the third order susceptibility. Such differences are brought about by the change of index of refraction due to the nonlinearity. Representative of this group of nonlinear interference devices is a bulk or guided wave Mach-Zehnder interferometer, although Sagnac interferometers, Michelson-type interferometers, distributed feedback grating devices, and Fabry-Perot resonators may also be included.
When these HMO glasses are used in synthesizing optical amplifiers, the gain coefficients for stimulated Raman and Brillouin amplification are also enhanced. This gain can be used to amplify a signal beam using a pump beam in a guided-wave geometry.
In areas where optical phase conjugation is desired, four-wave mixing interactions (bulk or guided wave) are utilized, wherein three input optical waves interact via the third order susceptibility to form a fourth wave, called a phase conjugate wave, which has unique properties. These properties can be exploited for such uses as aberration corrections, optical memory, beam steering, generation of new wavelengths, and neural networks.
In summary, the primary objective of the present invention was to design new glasses containing large concentrations of thallium, which glasses will exhibit a high degree of optical nonlinearity as well as superior visible and infrared transmission, those properties rendering the glasses particularly suitable for use in the aforementioned general areas of application.